Pros
Overall, F5 is a great company to work for. Those looking for a fast-paced tech company with opportunities to move up the corporate ladder will be disappointed, while those willing to "sink in" to the culture and put there time in will appreciate the relatively laid-back environment, knowledgable coworkers, excellent work / life balance, better-than-average salaries, and excellent benefits.
Cons
Product features obtained through acquisition have created a clear pecking order on the development side. If you're a dev looking for a chance to be recognized, the Seattle office -- specifically the BIG-IP team -- is where you want to be. If you find yourself in another office or on a team not related to BIG-IP, prepare to go unnoticed. For what it's worth, I worked on the BIG-IP team. Dev-side egos could definitely use a little reigning in, but his isn't unusual in the field. Opportunities for career advancement in product development are lacking, and the promotion process is inconsistent. There are *lots* of "old-timers" at F5. This speaks well for the work environment, but it can be frustrating watching middle-management make the same mistakes over... and over... and over again. These are smart people, but many choose to not rock the boat and simply follow the patterns established during their early days at F5.